1. Technical Field
The present invention relates to machine tools and more particularly to a machine tool which includes laser heat treating.
2. Background Art
Heat treating, metal surfaces with lasers is known. In American Machinist, May, 1976 pages 107 thru 111 and in SME Technical Paper MAR 75-570, both of whose teachings are herein incorporated by reference, heat treating and surface hardening with lasers is described in detail.
Transformation hardening is a solid state transformation of iron carbide in ferrous materials, at elevated tempertures, followed by a very rapid cooling rate to produce a hardened micro-sturcture. The temperature at which the solid state transformation takes place is approximately 1700 degrees F. Martensite is the iron carbide structure that is produced upon very rapid cooling from the transformation temperature. Martensite is the structure that produces the high hardness in ferrous materials. The rate at which the heated material is cooled from the transformation temperature is critical in transformation hardening. In conventional heat treating operations high cooling rates are achieved by chilling or quenching the workpiece with oil, water, or forced air. The faster the cooling rate the greater the degree of martensite formation and the higher the hardness. The carbon content of the material also determines final hardness; the higher the carbon content the greater its potential hardness.
CO2 lasers which have a high energy output are ideally suited for transformation hardening since they can provide for a very rapid heat input into the workpiece and extremely high cooling rates can be achieved as the heat is conducted into the workpieces by its own thermal mass.
Transformation hardening using a CO2 laser is accomplished by traversing the focused or defocused laser beam over a surface of the workpiece that has been prepared with an absorptive coating. Case depth obtainable with laser heat treating is a function of laser power, laser spot size, treatment speed, and the thermal properties of the workpiece material. The laser power and the laser spot size determines the power density of the process. The power density and the treatment speed are the two most critical parameters in laser heat treating. These parameters determine the actual heat input into the workpiece surface. The very fast heat input rapidly raises the surface temperature of the material above the transformation temperature but below the melting temperature.
Thermal properties of the material determine how rapidly the absorbed thermal energy is conducted into the interior of the workpiece. A rapid cooling rate promotes the metalurgical transformation of martensite. Cooling rates that exceed those of induction hardening are possible with laser heat treating. Laser heat treating therefore is capable of producing hardened cases of a few Rockwell points harder than those produced by induction hardening. The very rapid cooling rate that occurs in the material qualifies laser heat treating as a self quenching hardening process that does away with the need for an external quenching media.
At some depth the thermal energy dissipates and the temperature of the material does not rise above the transformation. This establishes the penetration depth of the hardened case. Increasing the power density or decreasing the treatment speed to promote a deeper penetration increases the risk of raising the surface above the melting temperature.
Since ferrous materials are normally reflective of laser energy, it is desirable to apply a coating to the workpiece surface to enhance energy absorption. Various coatings are available for this purpose, including magnesium, potassium, and zinc phosphates, indian ink, colliodal graphite, black spray paint, and black oxide coatings.
Turning machines having an NC control with indexable tool turrets supported for two axes movement are also known in the art. Typically, the tool turret is positioned by two stacked orthogonally positionable slides which are movable in response to commands from the controller. A machine tool as described in U.S. patent application Ser. No. 199,019 by R. D. Erickson, whose teachings are herein incorporated by reference, describes a machine tool having an indexable tool turret which is movable independently or simultaneously along an X axis and Z axis.